A short-circuiting compactMap for Swift's Sequence, that returns up to a maximum number of non-nil items

Sequence+CompactMapFirst.swift

/*
 * Sequence's compactMap lets you transform one
 * sequence of values into another, and strip
 * out any nil results. Sometimes, all you want
 * is the first non-nil result, and don't want
 * to futher evaluate the transform for the
 * remaining values in the original sequence.
 * This lets you do that.
 */
extension Sequence {
    func compactMap<ElementOfResult>(first maximum: UInt, _ transform: (Element) throws -> ElementOfResult?) rethrows -> [ElementOfResult] {
        var results = [ElementOfResult]()
        for element in self {
            if results.count >= maximum { break }
            if let result = try transform(element) {
                results.append(result)
            }
        }
        return results
    }

    func compactMapFirst<ElementOfResult>(_ transform: (Element) throws -> ElementOfResult?) rethrows -> ElementOfResult? {
        return try compactMap(first: 1, transform).first
    }
}

// Usage example:
let items = [1, 3, 7, 10, 15, 20, 21, 22, 23, 30, 34]
let transformTensMinusOne: (Int) -> Int? = { return $0 % 10 == 0 ? $0 - 1 : nil }

let tensMinusOne = items.compactMap(transformTensMinusOne)                          // -> [9, 19, 29]
let tensMinusOne_firstTwoArray = items.compactMap(first: 2, transformTensMinusOne)  // -> [9, 19]
let tensMinusOne_firstOneArray = items.compactMap(first: 1, transformTensMinusOne)  // -> [9]
let tensMinusOne_firstOne = items.compactMapFirst(transformTensMinusOne)            // -> Optional(9)